Optical devices that modify the properties of optical signals include devices such as tunable filters, attenuators, switches, polarization rotators and the like. Such devices use various means to periodically vary the refractive index of one or more regions of the device to change the phase/amplitude of a signal propagating through the device. Conventional devices of this kind include structures such as Bragg gratings and/or long period gratings to introduce the desired periodicity. Typically, conventional gratings are periodic perturbations in the photosensitive refractive index of the core of the optical fiber or waveguide. These gratings are created by UV exposure and are thus permanent in nature. Tuning of the applicable wavelength range may be achieved, for example, by introducing physical strain variations in the grating, temperature variations, magnetic field variations, or other environmental methods of inducing physical changes in the grating.
In a different scheme, it is desirable to have available an all-fiber device in which a periodic structure of a certain desired material (fluid/polymer/microspheres) is introduced into the fiber without the need for hydrogen loading or a photosensitive core. The introduction of the optically active material thus eliminates the need to use UV laser sources to write the grating structure, which is considered to save a significant amount of time and effort. Moreover, the choice of the active material's refractive index gives an additional degree of freedom in determining the difference in the refractive index perturbation, which is usually desired to be as large as possible so that coupling between different modes or different polarizations of one mode can be achieved in a relatively short coupling length.